|Bulkhead Grounding For Telecommunication
Most telecommunication facilities in the Land Mobile Services, Broadcast, Amateur
and Personal Services are woefully undergrounded. Each year insurance companies
pay out thousands of claims for lightning damage to installations that are "fully
lightning protected". When inspecting these facilities it's easy to see why
lightning damage is so easily caused, or why the station suffers from high noise
levels and lots of interference.
Proper grounding is almost
always the culprit, and it is so often the result of the ground system
being installed as a quick, cheap afterthought to radio equipment placement.
A really good engineer knows that the first item designed, constructed,
and installed in a telecommunication facility is the ground system. And
of the many ways that a grounding system can be employed, one type stands
out as most often the best performer. Better yet, it costs less to build,
involves less materials, and takes less time to implement.
Bulkhead ground systems
are very easy to understand. They consist only of a place and a single
metallic fixture. The bulkhead is most often a metal plate made from aluminum
but almost any metal will do just fine. A bulkhead could also be a piece
of heavy wire several feet long to which station cables and protective
devices are attached. It's not what you have, it's what you do with what
you have. Most installations consist of an outside antenna with a coaxial
line downlead connected to bonded equipment frames, and then a wire running
from that point to ground. This has the unfortunate effect of placing the
equipment chassis (and hence the operator if he is there) in series with
an incoming lightning surge, nearly always causing equipment damage and
often injuring the operator. A bulkhead grounding system overcomes this
deficiency by intercepting the incoming surges and shunting them to ground
BEFORE they reach the equipment.
A bulkhead plate is defined as a high integrty, zero or low inductance earth terminal
connection. For that reason a bulkhead plate should always be placed very close
to or on the ground with its lead connections to earth entry point very short -
preferably less than a foot long. All incoming lines to the facility (coaxial cable,
rotator lines, control lines, AC power lines, telephone lines, etc.) pass across
the plate where they are connected to various lightning protective devices. The
radio equipment should be located close by to take advantage of short distance grounding
to the bulkhead plate, thus reducing or preventing harmful local interference in
transmission and reception. No other grounds should be employed or lightning current
division may occur if the facility is struck. Division is what causes induced currents
to flow across equipment chassis, resulting in circuitry damage.
What makes a good bulkhead? A commercial relay rack panel of 1/8" thick aluminum
is quite nice and easy to obtain through electronic distributors. Copper plate is
ideal if available, but even a steel plate is acceptable. Mount antenna switches,
transmitting and receiving filters, and lightning protective devices all on the
same plate, and be sure to use an anti-oxidant between metal surfaces to ensure
good long term metal-to-metal electrical bonding. Examples of good anti-oxidants
are Burndy Penetrox, Ideal Noalox, or-I.C.E. Model 601 or 602. Mounting the bulkhead
indoors against an exterior wall is suggested so that access may be obtained and
protection from weather is assured. Keep all leads short and connections tight.
And try to obtain a plate that is bigger than what you expect to need so that some
room is left for facility expansion. Once you find out how nice the bulkhead system
works, you'll probably want to build a bigger station!